This invention relates generally to systems and methods of forming and shaping thermoformable material. More particularly, the invention relates to methods and systems for forming composite products from thermoformable materials.
Conventional composite parts composed of thermoformable materials, such as thermoplastic and thermoset materials, are well known and are widely used in the aerospace industry. Composites are preferred because they exhibit desirable characteristics, such as high strength and stiffness to weight ratios, corrosion resistance, and high temperature tolerance. Although highly desirable in the relatively non-cost driven military aerospace industry, composites are not widely used in other commercial industries because of the relatively high fabrication costs of composites, and the limited number of relatively non-complex shapes that are produced from known fabrication techniques.
Conventional fabrication techniques for forming composite parts include manual lay-up, matched-metal die forming, hydroforming, and diaphragm forming. The manual lay-up fabrication process requires an artisan to manually shape individual impregnated sheets or laminates (known in the industry as "prepreg sheets") to a desired form. Each successive layer is also manually shaped until a desired product having a selected number of prepreg sheets is formed.
The matched-metal die forming fabrication process employs a pair of complementary shaped and oppositely disposed metal dies that press-form a part from one or more polymeric resin impregnated sheets or laminates. Typically, the dies are machined to very tight tolerances to form a fixed mold gap, which houses the prepreg sheet during the lubrication process. The composite material is heated by the dies, or can be heated outside the die by any suitable means and then quickly transferred to the press, to form the composite product.
Similar to the matched-metal die forming technique, hydroforming also press forms a composite product from one or more prepreg sheets, but substitutes a fluid filled membrane for the top metal die. When a forming pressure is applied to the top membrane, the composite sheet and the fluid-filled membrane conform to the shape of a rigid bottom die. The prepreg is then heated to form a composite product.
Another traditional composite fabrication process is the diaphragm forming process which employs one or more diaphragms, rather than a fluid-filled membrane, to form the composite product. In this process, stacked prepreg sheets of a thermoformable material are placed between two diaphragms, typically formed of a thermoplastic material (e.g., polyimide films). The sheets are then pressure-forced against the shaping surface of a shaping tool by subjecting the prepreg sheets and the diaphragms to a differential pressure via a vacuum or other negative pressure source, thus forming the composite product. The resulting composite product can then be removed from the tool, and subsequently cured, if desired. Examples of the above techniques are described in U.S. Pat. No. 5,037,599 to Olson; U.S. Pat. No. 5,108,532 to Thein et al.; and U.S. Pat. No. 5,156,795 to Harvey et al.
A drawback of the foregoing manual lay-up technique, among others, is the relatively labor-intensive, and thus costly, manufacturing process required to form the composite product. Another drawback of conventional fabrication techniques is that they only produce composite products having relatively simple shapes, since the formation of more complex shapes is usually accompanied by an increase in the occurrence of undesirable deformations (e.g., wrinkling). The wrinkling reduces the overall strength and structural integrity of the composite part, as well as decreases the product's overall aesthetic appeal.
Hence, there still exists a need in the art for improved manufacturing techniques that can form composite parts having relatively complex shapes while reducing the occurrence of wrinkling therein. In particular, a method of forming a composite product that decreases costs associated with the fabrication of the part would represent a major improvement in the art. Additionally, a composite component that is relatively easy to manufacture and relatively free of wrinkling would also present a major improvement in the art.